Natural rubber is used for various products including industrial products such as tires, belts, and rollers, and sports products such as tennis balls. A rubber product is generally subjected to compression, recovery, and elongation repeatedly when used, which results in accumulation of loss energy to generate heat. The heat promotes rubber fatigue and thus shortens the life of the rubber product. A larger amount of the loss energy represented by loss tangent, tan δ, leads to a higher heat generation. Further, the rolling resistance of a tire is empirically known to be dependent on the value of tan δ at 50° C. to 70° C. This means that there are problems that a large value of tan δ causes an increase in not only heat generation but also in rolling resistance, and thus causes a decrease in fuel economy of a vehicle. Accordingly, it is desirable for a tread of a fuel-saving tire to have a small value of tan δ at 50° C. to 70° C.
Some Patent Documents disclose methods for reducing the protein and gel contents in natural rubber to decrease the value of tan δ of a rubber composition. For example, Patent Document 1 discloses a method of soaking solid natural rubber swollen with a solvent into an alkali hydroxide solution. Patent Document 2 discloses a method of removing magnesium phosphate by adding a phosphate to natural rubber latex. Patent Document 3 discloses a method of adding a protease and a surfactant to natural rubber latex and aging the latex. Patent Document 4 discloses a method of adding a surfactant to natural rubber latex and performing a washing treatment.
These methods can reduce the protein and gel contents to some extent, but not to a sufficient level. Also, deproteinization usually can reduce the protein content, but cannot sufficiently remove, particularly, phospholipids which are thought to be one of the factors of gel formation in natural rubber.
Meanwhile, natural rubber is usually modified in a latex state stabilized with a surfactant in consideration of the cost and the ease of handling, but is occasionally modified in a solid rubber state or in a rubber solution. However, natural rubber latex usually contains about 5% of a non-rubber component such as protein. Commercially available concentrated latex also contains about 3% of a non-rubber component. As a result, the non-rubber component, particularly protein, inhibits modification of natural rubber. This, for example, decreases the degree of grafting and grafting efficiency in graft copolymerization, which does not result in achievement of a high degree of modification and high modification efficiency.
In order to achieve high modification efficiency in that case, deproteinizing latex has been investigated. For example, Patent Documents 5 and 6 disclose a method of adding a protease to latex to degrade protein, and a method for producing a modified natural rubber by epoxidizing a deproteinized natural rubber that is produced by repeatedly washing latex with a surfactant. These methods can reduce the protein content to some extent. However, these methods cannot sufficiently remove phospholipids that are one of the factors inhibiting the modification of natural rubber, and there is still room for improvement.
Furthermore, there has been an approach to decrease rolling resistance of a tire to suppress heat generation and thus achieve fuel economy of a vehicle. The demand for achieving fuel economy of a vehicle from the aspect of tires has increased in recent years. The demand is particularly large for achieving fuel economy from the aspect of improvement of a tread which occupies a larger part of a tire than other tire components. Examples of known methods for achieving low heat build-up property of a rubber composition include a method of using a low-reinforcing filler, and a method of reducing the reinforcing filler content. Also, an attempt has been made to achieve fuel economy by using silica as a filler so as to decrease rolling resistance.
The above methods for achieving fuel economy from the aspect of the fillers decrease the hardness of the rubber composition, which softens the tire and problematically decreases the abrasion resistance. Hence, it is difficult to achieve both high fuel economy (low rolling resistance) and high abrasion resistance.
Those vehicle tires are subjected to a heavy load, and therefore the tires usually have carcass cords such as steel cords as reinforcements. The carcass cord can, however, be separated from the rubber composition particularly as a result of heat build-up of the tire while the vehicle is running, which can cause crucial tire failure. Hence, a rubber composition for covering a carcass cord needs to have high rubber strength and high adhesion to the carcass cord.
Rubber compositions for covering a carcass cord that have been used up until now contain natural rubber (NR) and/or isoprene rubber (IR) and emulsion-polymerized styrene butadiene rubber (E-SBR) as a rubber component, and contain carbon black as a reinforcing filler. This has led to a problem of poor fuel economy. In order to increase fuel economy, silica is generally used as a reinforcing filler in place of carbon black. Here, use of silica, however, decreases the required adhesion of the rubber composition for covering a carcass cord to the carcass cord, which makes it difficult to achieve both sufficient adhesion and high fuel economy (low rolling resistance).
In view of the above problem, Patent Document 7 discloses a method for achieving both sufficient fuel economy and adhesion by using a solution-polymerized styrene butadiene rubber containing a modified group that interacts with silica. However, only styrene butadiene rubber among diene rubbers is studied in the document, and natural rubber is not studied.
Natural rubber has a higher Mooney viscosity than those of other synthetic rubbers and thus has lower processability. Therefore, natural rubber is usually added with a peptizer and then masticated so that the rubber has a decreased Mooney viscosity before being used. Requirement of such a process in the case of using natural rubber decreases the productivity. Further, mastication causes molecular chain scission in natural rubber, thereby leading to a loss of the properties of a high-molecular weight polymer that natural rubber essentially has (for example, high abrasion resistance, fuel economy, and rubber strength).
Natural rubber latex is sap extracted from hevea trees and contains components such as water, protein, lipids, and inorganic salts as well as a rubber component. There is a report that removing protein contained in natural rubber improves the processability of the rubber. Some Patent Documents disclose methods for reducing the content of protein or another component in natural rubber. For example, Patent Document 1 discloses a method of soaking solid natural rubber swollen with a solvent into an alkali hydroxide solution. Patent Document 2 discloses a method of removing magnesium phosphate by adding a phosphate to natural rubber latex. Patent Documents 3 and 8 disclose a method of adding a protease and a surfactant to natural rubber latex and aging the latex. Patent Document 9 discloses a method of adding a surfactant to natural rubber latex and performing a washing treatment.
The methods disclosed in Patent Documents 1 to 3, 8, and 9 can remove some components such as protein to some extent, but not to a sufficient level. Also, there is a problem that these methods can hardly remove some components such as phospholipids. Further, studies have not been done on application of the natural rubber produced by these methods for a tire tread or a rubber composition for covering a carcass cord.    Patent Document 1: JP H11-012306 A    Patent Document 2: JP 2004-250546 A    Patent Document 3: JP 2005-082622 A    Patent Document 4: JP H06-329838 A    Patent Document 5: JP 2004-359773 A    Patent Document 6: JP 2005-041960 A    Patent Document 7: JP 2007-145898 A    Patent Document 8: JP H08-012814 A    Patent Document 9: JP 3294901 B